HD 191939 d
Astrophysical Synthesis & Habitability Characterization
HD 191939 d is a confirmed sub-neptune exoplanet orbiting the star HD 191939, located approximately 174.8 light-years (53.61 parsecs) from the Solar System. First announced in 2020 by Transiting Exoplanet Survey Satellite (TESS) using the Transit detection technique (producing an estimated primary transit depth of 0.0853% across the stellar disk), this world measures 2.995 Earth radii and has a measured mass of 2.8 Earth masses. Combining its mass and radius yields a mean bulk density of 0.57 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.31 g⊕ (3.04 m/s²), and an escape velocity of 10.8 km/s.
In its orbital architecture around HD 191939, HD 191939 d completes one full revolution every 38.353 Earth days at a semi-major axis of 0.2132 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 60.5 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.031 (ranging from 0.2066 AU at periapsis to 0.2198 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 540 K (+267°C / +513°F) and receiving 14.30 S⊕ (~19,462 W/m²), HD 191939 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 191939 is classified as a solar-type yellow star of spectral type G9 V with an effective photospheric temperature of 5,348 K, a radius of 0.94 R☉, a mass of 0.81 M☉, an estimated bolometric luminosity of 0.649 L☉. Stellar evolution modeling indicates an estimated stellar age of 8.7 Gyr and a metallicity of [Fe/H] = -0.15 dex (metal-poor relative to the Sun). From Earth, HD 191939 shines at apparent V-band magnitude 8.97 (accessible to binoculars and small amateur telescopes) in the Northern celestial hemisphere. The HD 191939 system is a multi-planet architecture hosting 6 confirmed worlds, with HD 191939 d sharing its stellar neighborhood alongside HD 191939 b, HD 191939 c, HD 191939 e, HD 191939 g, HD 191939 f.
Methodology: Computed using the Schulze-Makuch et al. (2011) weighted metric comparing physical interior (radius, estimated density) and surface thermal regime (escape velocity, equilibrium temperature).
Discovery Facility: Transiting Exoplanet Survey Satellite (TESS) · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 191939 |
| Spectral Type | G9 V |
| Effective Temp (Teff) | 5,348 K |
| Stellar Radius | 0.9400 R☉ |
| Stellar Mass | 0.8100 M☉ |
| Bolometric Luminosity | 0.649 L☉ |
| Metallicity [Fe/H] | -0.150 dex |
| Stellar Age | 8.700 Gyr |
| Apparent Magnitude | V: 8.970 · G: 8.775 |
| System Architecture | 1 Star, 6 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.213200 AU |
| Orbital Period | 38.353 days |
| Mean Orbital Velocity | 60.5 km/s |
| Orbital Eccentricity (e) | 0.03100 |
| Orbital Inclination (i) | 89.4900° |
| Mean Bulk Density | 0.57 g/cm³ (10.8 km/s vesc) |
| Insolation Flux | 14.3000 S⊕ (~19,462 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 174.800 LY (53.609 pc) |